1. Field of the Invention
The present invention relates to an electroluminescence (EL) display device, and more particularly, to an EL display device with improved external light coupling efficiency and brightness that can be easily manufactured.
2. Discussion of the Background
In an EL display device, the light emitted from a light-emission layer may pass through many layers before being emitted to the outside, and thus some light emitted from the light-emission layer is not emitted outside the device. The external light coupling efficiency ηex of an EL display device is given by the following equation:ηex=ηin·ηout where ηin represents the internal light coupling efficiency of the EL display device due to self-elimination of light within each layer. Further, ηout represents the output light coupling efficiency of the EL display device due to the variations in the refractive indexes of the layers and thus, the output light coupling efficiency represents the total internal reflection at the interface of the layers of the device. Specifically, total internal reflection at an interface occurs when the incident angle of a light photon is greater than a critical angle when the light is incident to an interface between a first medium with a higher refractive index to a second medium with a lower refractive index.
The fraction of the light emitted from the emissive layer that is ultimately emitted from the EL display device, defined as the output light coupling efficiency ηout or the light transmittance efficiency, can be expressed as:
                              1          2                ⁢                              (                                          N                out                                            N                in                                      )                    2                                    (        2        )            where Nin is the refractive index of the layer from which light is transmitted, and Nout is the refractive index of the layer into which the light is entering. For example, the output light coupling efficiency of light transmitting through a layer with a refractive index of about 1.5 to a layer with a refractive index of about 1.2 is determined to be 32% using Equation 2. That is, about 68% of the light which enters the interface is not emitted to the outside.
Because low external light coupling efficiency results in reduced brightness of the EL display device, there have been many attempts to prevent lowered external light coupling efficiency.
One method is to increase the supply voltage to the light-emitting layer. This method leads to improved brightness but requires increased battery capacity. Because increased battery capacity requires increased battery weight, the improved brightness resulting from this method is offset by the negative effect of the increased battery weight. In addition, the life of the battery and the EL element may be shortened. As a result, the following conventional methods have been suggested to improve luminescence while lowering the supply voltage.
Japanese Patent Publication No. hei 4-192290 discloses an inorganic EL device where a plurality of condensing microlenses of the same size or larger than an inorganic EL element are formed on an external surface of a transparent substrate, on which the inorganic EL element is formed. Light incident on the interface between the transparent substrate and air at an angle larger than the critical angle has an incident angle less than the critical angle on the microlenses, thereby reducing total internal reflection. Thus, the light is emitted in a predetermined direction to improve brightness of the device. However, the EL element is a surface light source in the cited invention, and thus diffused light that is not focused inevitably occurs when using a microlens that is the same size or larger than the EL element. In addition, clarity of an image displayed on the device is reduced due to overlapping images produced from adjacent EL elements.
Japanese Patent Publication No. hei 7-037688 discloses an EL element formed on a substrate with a cylindrically-shaped high refraction factor section formed of a material with a higher refraction index than the surrounding material formed around the side surface of the substrate. Light produced by the EL element is emitted through the high-refraction factor section to increase external light coupling efficiency. However, in the cited invention, the light which transmits through the high-refraction factor section is diffused light, as illustrated in FIG. 1 of the Japanese Patent Publication No. hei 7-037688. Thus, the brightness of the light emitted to the front is not significantly improved.
Japanese Patent Publication No. hei 10-172756 discloses an organic EL light emitting device in which one or more condensing lenses are disposed between a lower electrode of the organic EL light emitting device and a transparent substrate. The organic EL light emitting device and the condensing lens are positioned to correspond to each other. Light emitted from the EL light emitting device is incident on an interface of the transparent substrate at an angle smaller than the critical angle to increase the external light coupling efficiency. However, in the cited invention, overlapping images produced by adjacent EL light emitting devices reduce image clarity.